Hybrid electric vehicles (HEVs) differ from conventional motor vehicles in that they typically employ an electric machine operable as an electric motor in addition to an internal combustion engine in order to provide traction. HEVs are also typically equipped with a battery for storing electrical power for powering the electric machine.
In some HEVs the electric machine may also be employed as a generator under certain conditions. The electric machine may be powered by the internal combustion engine in order to generate electrical power to charge the battery.
It will be appreciated that HEVs represent a complex electrical-mechanical system the control of which is a non-trivial task. Considerable efforts have been made to develop controllers for HEVs that optimise the consumption of fuel by the engine in an effort to reduce the amount of carbon dioxide emitted by the vehicle during the course of a given drive cycle.
Some known parallel hybrid electric vehicles are configured to provide motive power to propel the vehicle by means of the engine only, the electric machine only or the engine and electric machine simultaneously. Peak performance figures in terms of torque delivered to the driveline are obtained by employing the engine and electric machine simultaneously. Use of the electric machine to provide torque to supplement that provided by the engine may be referred to as a ‘torque boost’ mode of operation.
It is to be understood that for each acceleration event in which the ICE and electric machine are used together, a significant amount of charge may be drained from the battery, reducing a state of charge (SoC) of the battery.
If acceleration events occur repeatedly in a relatively short space of time the state of charge of the battery may drop to a lower limit of its allowable operating range. This may render the electric machine incapable of producing motive torque. The absence of a torque boost contribution by the electric machine together with the added weight of electrical components associated with hybrid functionality cause a noticeable drop in vehicle performance.
In some known HEVs the problem of electric machine non-availability due to low battery SoC is resolved by providing a driver-selected hybrid-inhibit button which, when activated, prevents engine shutdown whilst the vehicle is being operated in order to recharge the battery as much as possible.
This solution, while protecting the performance figures, has a significant impact on fuel economy and vehicle emissions.